PHYTOCHROME ACTION AT HIGH PHOTON FLUENCE RATES: RAPID EXTENSION RATE RESPONSES OF LIGHT‐GROWN MUSTARD TO VARIATIONS INFLUENCE RATE and RED: FAR‐RED RATIO*

Abstract

Extensiongrowth rate of light-grown mustard (Sinapis alba L.) seedlings was monitored continuously using a sensitive linear displacement transducer system. When high flucnce rates (ca 2 mmol m-2 s-I) of mixed red and far-red light were presented to the growing internodes from fibre optic probes, fluctuations in extension rate occurred during the first 30 min. High red : far-red ratios (R : FR) caused growth deceleration, whilst low R : FR caused transitory growth acceleration. These changes in extension rate were not exactly as predicted from the proportions of Pr (the red-absorbing form of phytochrome) and Pfr (the far-red absorbing form of phytochrome) calculated to be established by the light sources. Nevertheless, the data demonstrate that phytochrome is able to control extension growth at fluence rates approaching those of summer sunlight, thereby providing the capacity to sense the presence of neighbouring vegetation before shading seriously compromises photosynthesis.Varying fluence rate over two orders of magnitude whilst maintaining R : FR constant evoked transient fluctuations in extension rate. At high R : FR, a 100-fold step down in fluence rate led, after a lag of ca 10 min, to a transient (i.e. 20 min) deceleration of extension that was followed by a marked transient (i.e. 20 min) acceleration. After a 100-fold step up in flucncc rate, a transient (i.e. 20 min) acceleration only was observed, beginning after a lag of ca 10 min. When R : FR was low, neither a step-down nor a step-up in fluence rate resulted in appreciable fluctuations in extension rate.The data are discussed in relation to the possible role played by the accumulation of photoconversion intermediates using a simple computer model for simulating active phytochrome concentrations at high fluence rates. The possibility that the mechanism for the photoperception of light quality by phytochrome may be capable of rapid adaptation to fluence rate fluctuations is proposed.

Genetic Evidence That the Red-Absorbing Form of Phytochrome B Modulates Gravitropism in Arabidopsis thaliana

“…Although there is a vast body of evidence (Kendrick and Kronenberg, 1986;Quail, 1991) that suggests that it is the Pfr form of phytochrome that plays an active role while the Pr form is inactive in phytochrome-dependent responses of a wide range of plant species, severa1 studies have led to the suggestion that both Pfr and Pr function to regulate stem elongation in light-grown plants (Smith, 1981(Smith, , 1983(Smith, , 1990). The results presented here, that normal negative gravitropism in Arubidopsis hypocotyls occurs when phytochrome B is in the Pr form (as indicated by the hy2 mutant) and is altered in the absence of phytochrome B (as indicated by the hy3 mutant) provide compelling evidence that the Pr form of phytochrome B is required for negative gravitropism in Arabidopsis hypocotyls.…”

Red, green and blue light photoreceptors controlling chlorophylla, biliprotein and total protein synthesis in the red algaChondrus crispus

“…Phytochrome in higher plants shows a fluence rate compensation, by which the relative proportions of Pr and Pfr at equilibrium are not affected by photon fluence rates between 50 and 150 lamol m -2 s -~ (Smith & Hayward, 1985;Child & Smith, 1987). However, low and high photon fluence rates can affect the rates of photoconversion of Pr and Prr At high irradiances, the photoconversion becomes limited by thermal conversion of the intermediates in the two pathways so that the last intermediates before Prr (meta-Rb) accumulate (Smith, 1990). Thus, photon fluence rate does not change Prr/Pt levels in higher plants but the rate of cycling between Pr and Pfr is affected (Frankland, 1986).…”

“…Thus, FR induction in Chondrus can not be explained by the typical action of phytochrome. Smith (1990) proposed that Pfr is not the only active form of phytochrome and the Pr must have some role in the control mechanism. In Chondrus, the level of Pr produced by FR could also have an important role in determining the effectiveness of both the induction and reversion of photoresponse.…”

Photoreceptors in Algae

“…(1984) proposed that changes in fluence rates rather than in light quality can act in twilight as time signals for photoperiodism. In populations of higher plants, some photomorphogenic responses occur in response to increased FR not only from the incident radiation but also from reflected FR (Bradburne et al, 1989;Smith, 1990). The latter effect provides the capacity to sense the presence of neighboring vegetation before shading seriously reduces photosynthesis .…”

“…In oceanic water blue light predominates in deep-waters, however, in coastal water the light is enriched in the green-orange region of the spectrum (Spence, 1981;Kirk, 1983;Anderson, 1986). In terrestrial habitats the plants detect the shading in canopies by measuring the ratio of R-FR light through phytochrome (Smith, 1982;Smith, 1990). Shade-habitats in canopies are increased in green and far-red radiation.…”